Boston’s Kendrick Perkins, left, and Kevin Garnett contained Atlanta’s Josh Smith in Game Seven of the first round.

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The Providence Journal / Glenn Osmundson

WALTHAM, Mass. — The Boston Celtics have been there and done that.

They will take on another lower-seeded opponent at the TD Banknorth Garden this afternoon in another Game Seven.

A victory and the Celtics will continue their quest of winning their first NBA championship in 22 years.

A loss, and Boston’s remarkable 66-win season will be diminished.

The Celtics made quick work of Atlanta in Game Seven in the Eastern Conference quarterfinals, but this time they are taking on a much better opponent in Cleveland. The Cavaliers posses one of, if not the best player in the NBA in LeBron James, who scored 32 points, pulled down 12 rebounds, and dished out 6 assists to lead the Cavaliers to a do-or-die Game Six victory.

“It’s a whole different monster,” Boston captain Paul Pierce said of Cleveland. “They are going to be a lot more mentally tough than the Atlanta Hawks were and we are going to expect a game out of them. They have experience. They’ve been to the finals. They’ve been in this situation before so they are not rattled and they’re not shaken by it.”

The Celtics know what’s at stake.

“This is what we played all year for and the reason why we have the best record in the NBA, for situations like this,” Pierce said. “We are a confident group, especially at home. We are playing well there throughout the playoffs but we have to play one more great game if we want to advance.”

Boston will be playing a confident Cleveland team this afternoon at 3:30.

“I’ll make sure my troops are ready for this Game Seven,” James said after Game Six. “We can win. We know we can win, so we just have to go out there and do it.”

Said Cleveland coach Mike Brown, “The group we have now, they’re tough, they’re resilient and they understand what it takes to win. We’ve faced a lot of difficult situations this year from the regular season on with this new group and our guys just found a way to win. We are looking forward to going to Boston and to have an opportunity to play there again and get a win on the road.”

Boston will draw a lot on its Game Seven experience against Atlanta. The Celtics won that game by 34 points.

“I think it helps us a lot,” Boston center Kendrick Perkins said of the Celtics having Game Seven experience. “You can see guys are more comfortable right now. In Game Seven against Atlanta, guys were uptight, screaming at each other. Now guys are kind of loose, supporting each other, and letting them know that we have to come out and play hard.”

Said Boston guard Eddie House, “I think it’s big for our psyche that we have had a Game Seven. We’ve got one under our belt. It’s so big that we are playing at home. We just have to take care of home again.”

Boston is 7-0 at the TD Banknorth Garden in the playoffs. The Cavaliers are 0-5 in Boston this season, but 2-1 all-time in Game Sevens.

“They get out and run extremely well at home [so] we can’t turn the ball over,” James said of Boston. “We can’t hurt ourselves…We can’t turn the ball over because it allows them to get extra offensive possessions. In this series, extra offensive possessions help because it’s such an offensive struggle for both teams because defensively both teams are locked in. You can’t go out there and turn the ball over and expect to win, especially on the road.”

The Celtics, who are 28-0 when leading a series, 3-2, and 15-3 at home all-time in Game Sevens, are averaging 87 points per game this postseason at the Garden against Cleveland. They are limiting the Cavaliers to 78 points per game.

“We feel like this is a game that we are going to go out there and win,” Pierce said. “It’s win or go home and we are right where we want to be. We have a great opportunity to advance to the Eastern Conference Finals. We’ve talked about it all year. This is where we want to be. This is what we played all year for.”

History will be on the Celtics side this afternoon. The Celtics are 18-5 all-time in Game Sevens. They held Atlanta to 29 percent shooting from the floor in Game Seven in the quarterfinal series.

“We’re confident,” Pierce said. “We felt like [Game Six] was a game that we should have won so coming into Game Seven, regardless of the scouting report, regardless of what we did today, we know pretty much everything about them. We just have to go out there and play and win the ball game. The guys are ready and focused.”

Containing James, running an efficient offense, and playing the suffocating defense that the Celtics have been known for this season are the keys to winning the game, Boston coach Doc Rivers said.

roblee@projo.com